Our objective is to increase our understanding of cellular mechanisms involved in implantation of the blastocyst. Implantation in the rhesus monkey is being investigated using light microscopy and both transmission and scanning electron microscopy, following perfusion fixation. Stages being examined include preimplantation embryos, initial adhesion of blastocyst to uterus, penetration of uterine epithelium and early post-implantation stages. Maternal aspects of the implantation process have focussed on the developing uterine epithelial plaque, indication of secretion by uterine glands, edema and inflammatory response in maternal tissues, and maternal vascular changes associated with the implantation site. In addition, studies of surface membrane components of trophoblast and adjacent uterine epithelium at implantation in non-primate laboratory animals are continuing.